Reducing the size of lasers and other photonic devices produces benefits similar to those seen with the shrinking dimensions of electronic components. A smaller laser requires less power and can potentially be switched on and off faster. In theory it is possible to use metals to form the laser resonator. The metal may form either strong compact mirrors, able to confine light to about the size of the diffraction limit. Alternatively, the light may interact strongly with the free electrons in the metal, being guided in the form of surface plasmon polaritons (SPPs) at the interface between a metal (conductor) and a dielectric (non-conducting) material such as air. Structures employing this form of light confinement have minimum size scales related to the penetration depth of light into the metal, which is typically of the order of tens of nanometers. Unfortunately, the metal conduction electrons which oscillate in synchronism with the optical field dissipate energy through collisions with the metal's atomic lattice. This energy dissipation leads to high optical losses, so SPPs can only travel short distances and metal mirrors have higher losses than dielectric ones. A number of workers have examined the possibility of using gain in metallic waveguides.
In the last couple of years efforts to use metals to form the nano-laser resonator has shown it is indeed possible for an optical gain medium to compensate the high optical losses. Furthermore, metals have allowed both the overall size of the laser to be reduced to smaller than the wavelength of light, and also the optical mode dimensions to be reduced below the diffraction limit. Some of these devices are coming close to being useful light sources, and it may only be a few years before we see lasers based on metallic nano structures in applications. Metallic or plasmonic nano-lasers could become technologically significant, particularly when the laser and mode size can be reduced far below the diffraction limit. Some of the major reasons for this significance are as follows: 1) There is a perceived need for small fast low power lasers to cope with the increasing bandwidth of inter and intra integrated circuit communications. 2) New applications may appear based on the ability to make arrays of coherent emitters with sub-wavelength pitch. 3) These metallic devices could offer improved light emitters compared to dielectric cavity devices, particularly for the longer wavelengths. 4) Ultra high speed low power lasers may open up the possibility for optical signal processing that is competitive with electronics in the high performance region. 5) Finally these devices will also be important sources and amplifiers for sub-wavelength plasmonic circuits.
The present invention is concerned with improved designs for the realization of metallic/plasmonic lasers and optical amplifiers based on MIM waveguide structures which include electrical pumping of the semiconductor gain medium. Electrical pumping is a key requirement for the practical application of small lasers based on metallic structures. In particular it is shown here that the proposed electrically pumped MIM waveguide design allows for the miniaturization of the semiconductor gain medium down to sizes where quantum confinement effects occur. Quantum confined semiconductor gain medium in one or more dimensions exhibits improved characteristics such as increased optical gain for a given carrier density, higher maximum optical gain, reduced temperature effects, higher differential optical gain resulting in possibly higher laser modulation frequency.
In the past there have been two major approaches to achieve lasing in MIM waveguides, and in particular those that propagate a plasmon gap or TM0 (Transverse Magnetic, lowest order) mode. The plasmon gap mode has the advantage that the size of the insulator between the two pieces of metal can be reduced to an arbitrary small size and still propagate a mode, permitting true deep sub-wavelength confinement of light. The first approach has been used in the past for wavelengths in the mid and far infra-red (approximately >3 microns). Here, the semiconductor gain medium is sandwiched between two pieces of metal which form the electrical n-type and p-type contacts. Generally a thin layer of titanium or other adhesive metal is used between the semiconductor and gold to assist adhesion of the metal to the semiconductor. Also there are often specific doped semiconductor layers close to the metal to achieve a low contact resistance between the metal and semiconductor.
A typical prior art MIM waveguide structure employing this approach is illustrated in FIG. 1, which illustrates in schematic cross-section a MIM waveguide containing electrically pumped semiconductor, used for long wavelength semiconductor lasers. This form of electrically pumped MIM waveguide represents the state of the art when the semiconductor core is thick (>1 micron), and the wavelength of light is long.
For the longer wavelengths at which such devices operate, the metal losses are low and the extra losses due to the titanium layer are acceptable. Furthermore the semiconductor layer between the metal is typically several microns thick. The thickness of this layer permits the use of n and p doped contact layers in the semiconductor, while still having a significant proportion of the semiconductor as undoped gain medium. For shorter wavelengths, and when the size of the semiconductor region is to be reduced down to a thickness of a few hundred nanometers or less, then this simple approach of making direct electrical contacts to the semiconductor with the confining metal becomes problematic. The problems arise due to the following factors: adhesive metals such as titanium introduce significant optical losses at these shorter wavelengths and so, are difficult to employ. Furthermore to reduce optical losses of the noble confining metal and also to improve the contact resistance of the metal to the semiconductor, the noble metal is often annealed at high temperatures. When the metal is placed directly on the semiconductor, this annealing process typically causes elements of the semiconductor to dissolve into the noble metal, and the noble metal to penetrate into the semiconductor PIN junction. Apart from disturbing the purity and crystal structure of the noble metal, the actual PIN junction can be short circuited.
Currently small electrically pumped MIM waveguide plasmon mode lasers operating at near infra-red wavelengths (˜1.5 microns) are realized with a different concept, as illustrated in FIG. 2, [1], [2], [3], [4]. FIG. 2 is a schematic cross-section of a MIM waveguide with electrically pumped semiconductor which permits small core width (90 nm) and short wavelength operation. Here a rectangular cross-section pillar is etched into a complex semiconductor heterostructure. The middle of the pillar structure contains a high index, lower band-gap semiconductor, in this case InGaAs (Indium gallium arsenide). A slightly lower index, higher bandgap semiconductor (here InP) surrounds the center region. The pillar is coated in a thin dielectric layer, and then encapsulated in metal. The plasmon gap mode is only weakly localized in the middle of the pillar waveguide, overlapping the InGaAs, due to the slightly higher refractive index of the InGaAs. InP has a refractive index of 3.17, while that of InGaAs is slightly higher (14%) at 3.6.
Effectively the MIM structure is turned 90° onto its side. At the top of the pillar the dielectric layer is removed and special contact metals and semiconductor are used to make a good electrical contact. Around the base of the pillar adhesive metals are also used to bond the metal noble metal to the semiconductor structure. The other electrical contact is made off to the side of the device, and current can flow through the substrate, up the InP below the InGaAs, into the InGaAs gain material. The other top contact completes the circuit through the top InP layer.
In essence the MIM waveguide structure of FIG. 2 consists of two metal layers with a core (the layer between the two metal layers) comprised of semiconductor. The MIM structure consists of three distinct waveguide sections: A middle section containing the lower bandgap semiconductor (InGaAs) core; and, either side of the middle section are two sections of MIM waveguide which contain a higher bandgap semiconductor (InP). The contrast between the effective refractive indices of the waveguide sections is only slight due to the similar high refractive indices of the semiconductor materials (InGaAs and InP).
The waveguide sections also contain two thin dielectric layers between the semiconductor materials and the metal. These layers are made as thin as possible, and with as high a refractive index dielectric as possible, to maximise the energy of the optical mode confined in the InGaAs region [2]. In the prior art realised devices, SiN with a relatively high refractive index for a dielectric of 2 was used.
With the structure of FIG. 2, a number of important problems are solved. The electrical contacts are located well away from the optical mode, allowing the use of low bandgap semiconductors, low contact resistance metals, and annealling to make good, reliable contacts. Near the optical mode, the metal is separated from the semiconductor by a dielectric layer allowing high temperature annealling to improve the optical loss of the metal. Furthermore, near the optical mode there is only the low optical loss noble metal. However, as the active region in this particular structure is reduced in size, limitations in the approach appear.
FIG. 3 shows an idealized cross-section for such a waveguide with the height (h) of the InGaAs region set at 90 nanometers (nm) and the width (W) of the InGaAs region varied from 300 to 20 nm. The width of the insulator on the sidewalls of the pillar is 5 nm and the refractive index of the insulator is 2. The active semiconductor region is InGaAs, while the higher bandgap semiconductor InP. SiN (Silicon Nitride), which has a relatively high refractive index for a dielectric of 2, is used as the insulating dielectric layer between the semiconductor and the metal, which here is silver. The InGaAs/InP semiconductor materials are useful for wavelengths in the region of 1.5 microns, however other compound semiconductor materials could be used to cover different wavelength regions, or give different characteristics.
A mode solver is used to numerically calculate the electric field profile of the electromagnetic mode that propagates in the waveguide. From such a mode profile the overlap of the electric field with the InGaAs gain medium and the metal can be found, and from these values the amount of optical gain required from the InGaAs gain medium to overcome optical losses from the metal can be calculated. The gain required to overcome losses and also the percentage of the mode energy contained in the InGaAs gain region are plotted in FIG. 4. This gain is important to know as it determines if it is possible to make a laser or amplifier with the waveguide. The gain as a function of InGaAs width is shown in by the solid curve.
From FIG. 4 it can be seen that as the width of the active region is reduced down to a few tens of nanometers, the gain required to overcome losses increases greatly (solid curve in FIG. 4) and also the amount of modal energy in the gain region reduces. The high gains required for the smaller widths would be difficult to achieve with semiconductor gain materials. Thus for smaller widths, this waveguide concept is not suitable for lasers or amplifiers. The percentage of modal energy in the InGaAs versus InGaAs width is shown by the dashed line in FIG. 4. It can be seen that for small widths, there is very little energy in the InGaAs. The key problem with this prior art waveguide design [1], [2], [3], [4] is that as the width of the waveguide is reduced, the energy of the mode goes into the dielectric layer and is poorly confined on the InGaAs region.
In general, to make useful devices such as lasers or amplifiers, it is desirable to have the gain required to overcome metallic losses as low as possible, along with a large proportion of the modal energy overlapping the gain medium. The lower gain will translate to lower threshold currents, and the higher proportion of energy in the gain medium will allow for a higher net amplification along the metallic waveguide. The higher net amplification can result in smaller and more efficient lasers and amplifiers.
The present invention was developed with a view to providing a MIM waveguide structure with improved design that is less susceptible to the above-noted problems with prior art MIM waveguide structures.
References to prior art in this specification are provided for illustrative purposes only and are not to be taken as an admission that such prior art is part of the common general knowledge in Australia or elsewhere.